Hollow Nanoreactors Modulate Mass Transfer and Dual-Species Spillover to Boost Nitrate-to-Ammonia Reduction in Real Wastewater.

Copper-based materials are promising catalysts for electrocatalytic nitrate reduction to ammonia (NH3), while their use in real wastewater with low nitrate concentrations is hindered by poor mass transfer and high energy barrier. To overcome these limitations, we design a hollow Cu/MoS2-550 nanoreactor, consisting of hollow MoS2 support loaded with Cu single atoms and clusters. At the mesoscale, the hollow MoS2 support features a heat-exchanger-fin-like structure that accelerates mass transfer, thereby promoting local enrichment of NO3 -. At the microscale, precise modulation of sulfur vacancy concentration in MoS2 triggers dual-species spillover, namely reverse hydrogen spillover and *NO spillover from Cu single atoms to Cu clusters, which lowers the energy barrier of deep hydrogenation step. As a result, the Cu/MoS2-550 nanoreactor achieves an NH3 Faradaic efficiency (FE) of 98.14% and a yield rate of 27.46 mg h-1 mgcat -1. Furthermore, when assembled into an Al-NO3 - battery operating in real wastewater containing only ∼ 0.76 mM NO3 -, the battery runs stably for 120 h, delivers an NH3 FE of 53.20%, and maintains a nitrate removal rate of 91.17%. This work provides cross-scale modulation strategies to overcome mass-transfer bottlenecks and energy barriers in multi-electron transfer reactions, offering a potential pathway for environmental applications.